A conventional zigzag sewing machine has an arm including a needle bar drive mechanism that vertically moves a needle bar having a sewing needle attached thereto and a needle swing mechanism that swings the needle bar; and a sewing machine bed including a needle plate and a feed dog that projects/retracts from the needle plate in order to move a workpiece cloth.
The feed dog is generally provided with a plurality of longitudinally extending teeth, and is fed in a four-step cycle by a cloth feed mechanism provided inside the sewing machine bed. Formed on the needle plate disposed on the sewing machine bed upper surface is a needle hole extending laterally so as to correspond to the lateral swing of the sewing needle; and a plurality of longitudinally extending square holes for projecting/retracting a plurality of teeth therethrough.
The zigzag sewing machine of the aforementioned type moves the feed dog in plurality directions to provide for sewing complex sewing patterns. For example, JP-Y1-S54-135263 discloses a cloth feed dog having four rows of teeth which not only perform four-step feed, but also move in the lateral direction as well. Under such construction, the widths of the four rows of square holes are widened, allowing the teeth to move in the lateral direction. Thus, the workpiece cloth is moved by the teeth in both longitudinal and lateral directions, as well as in the oblique direction which is a combination of the aforementioned two directions.
However, under such construction, there is an increased possibility of defective stitches being formed upon forming straight stitches on the workpiece cloth. That is, since the widths of the square holes are widely arranged with respect to the four rows of teeth, depending upon the position of the needle drop point, the cloth presser and the teeth fail to clamp the straight stitches formed on the workpiece cloth. Hence, there are increased instances where cloth feed is carried out with a stitchless portion of the workpiece cloth being fed by being clamped between the cloth presser and the teeth. At this time, since tension is applied on a needle thread by cloth-feed, seam puckering is observed on the stitches of the workpiece cloth, forming defective seams in which the stitches are drawn up. Defective seams become more prominent especially upon sewing a thin workpiece cloth, or sewing with thicker sewing thread or with increased sewing speed. Reducing the thread tension of the thread tension regulator is a possible solution to the above problem. However, in such case, repetitive trial sewing needs to be performed in order to obtain the suitable thread tension for various types of workpiece cloth, sewing thread and sewing speed. This calls for a complex and time consuming preparatory work, which does not satisfy practical use.